The present invention relates to filters, and more particularly, to integrated circuit active filters.
In general, a filter is a two port network which is designed to freely transmit signals within one or more frequency bands and to attenuate signals of other frequencies. Conventional filters comprise networks of resistors, inductors and capacitors. The recent availability of integrated circuit (IC) operational amplifiers (op amps) has made it possible to simulate the electrical characteristics of R-L-C networks by using only resistors and capacitors. An op amp with capacitive feedback can be designed to act like an inductance or a complete L-C network. This approach eliminates bulky inductors and ferromagnetic effects. Filters of this type are called active filters. Their main advantages are that they can be designed to have no insertion loss, and that they can be constructed in much smaller volumes than their passive counterparts.
The op amp in an active filter is a high gain amplifier which has an inverting and a non-inverting input. The ideal op amp has infinite input resistance, infinite gain and zero output resistance. However, in actual IC's these characteristics cannot be fully achieved. Nevertheless, IC op amps are available that provide very high input impedance, very large amplification and very low output impedance over a useful frequency band. Active filter design involves the application of modern network theory. Usually, either Butterworth or Chevishev filters are implemented, although other filters can also be obtained.
Low pass, high pass and band pass filters are classified into degrees of complexity. In general, second order filters are considered the simplest. The signal inverting multiple feedback (MFB) and the nonsignal inverting voltage controlled voltage source (VCVS) are widely used circuits in active filters.
In the past, there has been available from National Semiconductor Corporation, the assignee of the present application, a general purpose second order active filter sold under the designation AF100. Four external resistors program the AF100 for specific second order functions. Low pass, high pass and band pass functions are available simultaneously at separate outputs. Notch and all pass functions are available by summing the outputs in the uncommitted output summing amplifier. Higher order filters are achieved by cascading several AF100 active filters with appropriate programming resistors. Any of the classical filter configurations, such as Butterworth, Bessel, Cauer, and Chevishev can be formed with the AF100.
The aforementioned AF100 active filter represents an improvement over conventional filter design in that it does not utilize any inductances. Furthermore, it comes in one convenient package that can be readily programmed with external resistors to provide a wide range of useful second order filter functions.
Prior art active filters have frequently been difficult to tune. Furthermore, they have had a hybrid construction incorporating both IC and discrete components. This has made them relatively large in size and high in cost.
It would be desirable to provide a monolithic IC general purpose active filter that could be readily programmed for specific second order functions. If such an active filter could perform an all pass function, it could be utilized for phase correction in a wide variety of signal processing applications, for example, in high speed modems used in communications systems. Such a monolithic IC general purpose active filter could be produced at relatively low cost and would be relatively small in size. Preferably, this monolithic active filter would have improved tuning capabilities.